Seeing sizes in the sky doesn't mean much, one man's 16 is equal to anothers 20m. The only time it bears relevence is when you kite at an unfamilar spot & there may be wind shielding in shore. Then the only way to know for sure is to see who is lit & who is a dickhead out there out of control.!!

It is just another piece of useful information to add to the mix! every bit helps. It was like a culture in windsurfing checking the sail sizes, whos on what, its all good I like seeing the kite sizes at any part of the day.

I always like to see what other guys are on before setting up to launch. Beyond that I like to see and ask for that matter how they are doing for their weight. Are they overpowered with the kite flying near the vertical offshore or are they having to heavily sinusoid the kite? I need to get a feeling for wind characteristics for tuning the kite during setup. I also need to know if it is necessary to walk back to the car and swap out for another kite size. It is often quite hard to read kite sizes from shore. Larger size numbers would be a help. All this despite checking realtime internet winds and doing hand anenometer wind checks particularly at the lower end of things.

The only concern that I would have about this is that it can lead to 'deferrence of risk' where someone says xxx is on a 12m so I'll rig one.
We all need to decide for ourselves.
I usually decide by 'feel', then take a wind reading and then ask a rider 'how it feels out there'. Sometimes you will be told that 'there's more wind out there' or something that could help you make a decision if you are between kite sizes.
Line length also comes in to it. I ride my R4 20 on 20m lines. Sometimes I'm still riding it when other people are on 12's (usually when I've been out for a while and it's been picking up). While this MIGHT be a reasonable choice for me, I'd hate to watch someone else put a 20 up just because they've seen the sticker on my kite.